In order to begin the combustion process inside a fossil fuel fired combustion chamber, such as that found in industrial and utility boilers, it is desirable to have an energy source to begin the self-sustaining combustion reaction of fuel and air inside the combustion chamber. Current practice is to use a light fuel oil, natural gas, or propane ignitor of a size between input of 0.5 to 20 Million Btu/hr for each of several fuel admission compartments of the combustion chamber.
Ignitors have a dedicated fuel and air supply and an energy source, typically a spark plug, to produce a flame. In operation, fuel and air are introduced to the ignitor and a spark provides the energy to begin a self-sustaining reaction that keeps the ignitor burning. Proof that the ignitor is operating is established through the use of a flame detector, such as a flame rod, a thermal sensing device, or an optical sensor, that is often integral with the ignitor.
Once the ignitor is proven to be operating, main fuel and air for the combustion chamber can be introduced, often after utilizing the ignitor to preheat the combustion chamber. The energy from the ignitor (the ignitor flame), allows the combustion reaction of the main fuel and air to begin. Generally, once the main fuel and air is ignited, the combustion reaction is self-sustaining and the ignitor can be turned off. However, in some cases, such as due to low volatility of the main fuel, it is necessary to leave the ignitor on in order to keep the main combustion reaction continuing. In other cases, ignitors are left to burn continuously, as may be required by safety laws.
For reasons of safety it is important that the ignitor reliably begin burning on command, and that it be able to be confirmed that the ignitor is producing a flame to insure the safe combustion of the main fuel and air. Failure of an ignitor can result in unsafe accumulations of unburned main fuel and air, resulting in massive explosive damage.
In one type of coal-fired boiler unit, one or more relatively high-capacity oil burners (warm-up guns) are started by one or more oil- or gas-fired ignitors to preheat the combustion chamber. Once the combustion chamber has been brought up to the proper starting temperature, coal nozzles are ignited by the oil- or gas-fired ignitors, or by the warm-up guns themselves.
At higher boiler loads, i.e., when the amount of coal supplied by the coal nozzles is great, the combustion chamber can typically maintain stable combustion of the pulverized coal. However, when the load goes down and the coal supply is thereby decreased, the stability of the pulverized coal flame is also decreased, and it is therefore a common practice to use the ignitors or warm-up guns to maintain the flame in the combustion chamber, thus avoiding the accumulation of unburned coal dust in the combustion chamber and the associated danger of explosion.
Certain portions of an ignitor mounted in a windbox compartment of a combustion chamber are subjected to relatively high temperatures, typically on the order of 500 degrees Fahrenheit or higher. In some conventional ignitors, there is a risk that an ignitor wire supplying energy to an ignitor spark element may burn up due to the high temperatures, especially when insufficient cooling air is supplied to the ignitor.
An ignitor's spray of fuel and air (the combustive mix) is produced by an atomizer. The spray produced by conventional atomizers used in oil-fired ignitors frequently has too many large droplets, resulting in insufficient oxygen at the base of the flame. An insufficient amount of oxygen results in excessive smoke formation, resulting in an unacceptable opacity from the stack.
Introduced above, conventional ignitors, no matter the type of ignitor fuel utilized, include some sort of flame sensing device which may be mechanical or optical. The output of such a flame sensing device is transmitted to a control room where operational decisions are made based upon the sensed flame. If no ignitor flame is detected when one is expected to be present, repair personnel begin servicing the non-performing ignitor based upon only the information that a flame is not present. Lack of a flame could be due to any one of a faulty ignitor fuel supply, a faulty ignitor compressed air, or a faulty ignitor spark source. Further, a flame could actually be present, and the flame detector itself could be sending a false lack of flame signal.
The FIG. 1(A) depicts one embodiment of an existing commercially available ignitor spark and flame rod systems 200 mounted in one of the windboxes of the fossil fuel-fired steam generator (not shown). The FIG. 1 shows two rods—a flame rod system 210 and a spark extension assembly system 215. The ignitor spark and flame rod systems 200 is mounted inside a conduit 201 secured to a windbox wall 205. The ignitor spark and flame rod systems 200 includes a flame rod system 210, a spark extension assembly system 215, a compressed air conduit 225, a fuel conduit 230 collinear and disposed within the compressed air conduit 225, a bluff body 240 disposed at the terminus of the compressed air conduit 225, and an atomizer 235 disposed within the bluff body 240.
The spark extension assembly system 215 includes a solid conductor with an outer ceramic insulation coating, enabling the spark extension assembly system 215 to survive temperatures greater than 1000 degrees Fahrenheit. The spark extension assembly system 215 also contains electrical circuitry shown in the FIG. 1(B) and discussed in detail below. The solid conductor, preferably made of stainless steel, though it could be any other conductive metal, connects to an external electrical power source (not shown in the Figures) at terminus 255. At the opposite end of the spark extension assembly system 215 is a high energy ignitor tip 220. The solid conductor receives electrical current from the power source and conducts the electrical current to the high energy ignitor tip 220, which produces a spark to ignite a spray mixture of the compressed air and fuel released by the atomizer 235. The compressed air conduit 225 facilitates the delivery of compressed air to the high energy ignitor tip 220. The use of compressed air facilitates rapid ignition of the compressed air-fuel mixture when contacted by the spark. The high energy ignitor tip may be a spark plug or alternatively, it can be button of metal welded to the solid conductor. The spark occurs between the button and the ground (also referred to herein as electrodes). The button allows for precise positioning of the spark.
FIGS. 1(B) and 1(C) detail the electrical circuitry for the spark extension assembly system 215 and for the flame rod system 210 respectively. The spark extension assembly system 215 is used to initiate a flame in the furnace by generating a spark across a pair of electrodes that are disposed in the furnace. With respect to the FIG. 1(B), the electrical circuitry for the spark extension assembly system 215 includes an alternating current power source 252 in electrical communication with a spark transformer 250 that comprises a primary winding 254 and a secondary winding 256. The secondary winding 256 is in electrical communication with the spark extension rod 216 that in turn communicates with high energy ignitor tip 220 that comprises two electrodes. As seen in the FIG. 1(B), the two electrodes are separated from each other by an air gap, with the electrode that is not in direct electrical communication with spark transformer being grounded. The low voltage end of the secondary winding 256 is also grounded.
The alternating current power source 252 generates an electrical current that is transmitted to the high energy ignitor tip 220 via the spark transformer. This electrical current is generated upon manual actuation (because it is desired to start combustion in the furnace). Because of the high voltage (e.g., greater than 1000 Volts, preferably greater than 5000 Volts), a spark is created in the air gap between the two electrodes at the high energy ignitor tip 220. The spark ignites the air-fuel mixture in the furnace (not shown) thereby permitting combustion of the air-fuel mixture.
The electrical circuitry for the flame rod system 210 is depicted in the FIG. 1(B). The flame rod system 210 is used to indicate a lack of a flame during operation of the furnace. The flame rod system 210 comprises a flame monitoring rod 270, a flame monitoring sensor 265, a flame monitoring ignitor 272 and a ground contact 274, all of which are in electrical communication with one another.
In operation, the flame rod system 210 is charged to approximately 40 volts DC, allowing for an optimum signal-to-noise ratio. As flame ions interact with the flame monitoring rod 270, the voltage dips and rises. These voltage fluctuations are measured by a flame monitoring sensor 265. The flame monitoring ignitor 272 is a complete ignition system containing an electrical spark source, a self-stabilizing burner device, flame detection, and fuel input monitoring systems. The flame monitoring ignitor 304 takes advantage of the production of ions and charged particles during the combustion of hydrocarbon fuels. Due to the presence of these particles, a hydrocarbon-fuel flame will conduct electricity.
When a DC potential is placed across a flame, the electric current flow varies at the same frequency as the flame pulsation. The flame monitoring ignitor 272 operates by imposing a DC potential on an electrode called the flame rod, which is in contact with the flame. When there is “no flame,” the DC voltage remains at the originally imposed level, and no current flows. When there is “flame,” the DC voltage drops as current flows, generating an AC feedback signal. This AC signal is filtered, amplified, and modified by the flame monitoring ignitor electronics to drive a flame indication relay. If a component failure occurs (e.g., a short circuit in the flame rod or signal lead wire, or an external AC interference), a “no flame” indication will occur. The indication of a “no flame” signal generally leads one to actuate of the spark extension assembly system 215 via a switch (not shown), which restarts the ignition process.
As seen from the FIG. 1(A), most existing commercially available ignitor spark and flame rod systems have two or more protruding rod assemblies for spark ignition and for flame sensing and proving. These protruding rods are sometimes identical in appearance. Each of these protruding rods uses internal stand-offs, an external connector assembly, an external wire train and requires dedicated conduit and wire to be run back to the ignitor control cabinet.
Elimination of some of these supporting elements for two protruding rods would reduce manufacturing costs, increase manufacturing speed and also eliminate some of the problems detailed above that are associated with the various ignitor functions.